Yes, mitt is the singular form; mitts is the plural form.
Yes. Hay is a singular noun. A Singular noun means one item only. So technically, hay is a singular noun.
The noun 'theory' is a singular noun. The plural noun is 'theories'.
The noun 'mice' is the plural form of the singular noun 'mouse'.
Yes the word question is a singular noun. The plural noun is questions.
The noun mailbox is the singular form; the plural is mailboxes.
Feet is a plural noun. Foot is a singular noun.
The noun juggler is a singular noun. The plural noun is jugglers.
The noun school is the singular form.The plural noun is schools.
The noun 'fun' is a singular non-count noun, it has no plural form.
The singular noun is "louse" and the plural noun is "lice."
The noun family is the singular form; the plural form is families.
The noun children is the plural form of the singular noun child.